Diagnostic efficacy of contrast-enhanced fluid-attenuated inversion recovery (FLAIR) imaging in idiopathic cerebrospinal fluid rhinorrhea

We report a case of a 50-year-old woman in which contrast-enhanced fluid-attenuated inversion recovery (FLAIR) was used for the diagnosis of idiopathic cerebrospinal fluid rhinorrhea. The pre- and postcontrast FLAIR subtraction images showed a contrasted protrusion of the right olfactory cleft canal, highlighting the potential practicality and effectiveness of using pre- and postcontrast FLAIR subtraction images in diagnosing idiopathic cerebrospinal fluid rhinorrhea, in conjunction with conventional high-resolution computed tomography and magnetic resonance cisternography. The successful diagnosis of cerebrospinal fluid rhinorrhea allowed for treatment through endoscopic nasal surgery to close the fistula with a positive clinical outcome.


Introduction
High-resolution computed tomography (CT) and magnetic resonance (MR) cisternography are conventionally used for ✩ Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
imaging diagnosis of idiopathic cerebrospinal fluid (CSF) rhinorrhea [ 1 ,2 ].Contrast-enhanced MR cisternography has also been reported to be beneficial for CSF rhinorrhea diagnosis [ 3- be identified, it is not necessarily complicated by a dural release [ 2 ,7 ].MR cisternography has been reported to have high sensitivity and specificity (80%-90%) for the diagnosis of CSF rhinorrhea but is inferior to contrast-enhanced MR cisternography [ 4 ,5 ].Contrast-enhanced MR cisternography has high diagnostic performance but requires intrathecal contrast administration and is an invasive test.On the other hand, postcontrast FLAIR has been reported to be useful in diagnosing lesions bordering the CSF [ 8 ] but has yet to be evaluated as a potential tool in diagnosing CSF rhinorrhea.Here, we report a case in which a pre-and postcontrast FLAIR thin slice subtraction image, combined with high-resolution CT and MR cisternography, was used to diagnose CSF rhinorrhea.

Case report
We report a case of persistent watery rhinorrhea in a woman in her 50s.The patient experienced a fever following coronavirus vaccination.Watery rhinorrhea coincided with fever onset.Her previous physician suspected allergic rhinitis and commenced treatment.However, no improvement was observed, and the patient was referred to our otorhinolaryngology department.The nasopharyngeal fiberscope revealed bilateral nasal mucosal swelling and soluble nasal discharge from the right nasal cavity.The 3-mm-slice CT scan showed no sinusitis or CSF rhinorrhea.Based on the above, severe allergic rhinitis was suspected, and drug treatment and posterior nasal nerve transection were performed.However, the persistent right-sided soluble rhinorrhea during forward bending led to reconsideration of CSF rhinorrhea.An intranasal glucose concentration test was performed, which showed a high value of 58 mg/dL (normal range: < 30 mg/dL).CSF rhinorrhea was suspected, and the MR cisternography scan was performed.At that time, the initial CT was also reviewed.A 0.5-mm slice reconstruction of the initial CT before the MRI scan revealed a soft density protrusion of the right cribriform plate of the ethmoid bone ( Fig. 1 ).MR cisternography also demonstrated CSF pooling in the right nasal cavity through the defect in the same area.However, the findings were minor, making it challenging to determine the pathology ( Fig. 2 ).To further clarify whether these findings were responsible for the patient's ongoing watery rhinorrhea, FLAIR 2-mm slice imaging was conducted both pre-and postcontrast to create a subtraction.The FLAIR imaging displayed a clear contrast effect on the periphery of the CSF pooling in the right nasal cavity ( Fig. 3 ).This finding suggested dural disruption and vascular injury at the same site, leading to the diagnosis of CSF rhinorrhea.Endoscopic nasal surgery was performed, and the fistula was closed ( Fig. 4 ).The postoperative course was successful, and no recurrence was observed 6 months after surgery.

Discussion
FLAIR is a special pulse sequence that uses inversion recovery with a long repetition time (TR) and echo time (TE), as well as an inversion time (TI) that effectively eliminates signals from the CSF [9][10][11].Although FLAIR images are T2-weighted images (T2WI), the contrast effect in contrast-enhanced FLAIR is due to the mild T1 shortening effect caused by the long TI.Additionally, FLAIR not only suppresses CSF but also the intravascular signal in normal blood flow, resulting in a more noticeable contrast effect in the lesion [ 12 ].This is a feature not observed in postcontrast T1-weighted images (T1WI).Therefore, it has been reported that contrast-enhanced FLAIR is useful in diagnosing lesions bordering the CSF.In this report, FLAIR is more sensitive to T1 shortening than T1WI at lower gadolinium (Gd) concentrations [ 8 ].However, the hyperintensity lesions observed with contrast-enhanced FLAIR alone may be caused by either T2 lengthening or T1 shortening.Therefore, both pre-and postcontrast imaging are necessary, and preand postcontrast subtraction may be useful [ 13 ,14 ].Based on these reports of contrast-enhanced FLAIR, we applied preand postcontrast FLAIR subtraction to detect CSF rhinorrhea.The pronounced differences in contrast observed in contrastenhanced FLAIR in our case may be due to trace amounts of Gd contrast agent leaking from the dural disruption and vascular injury in the cribriform plate of the ethmoid bone.Currently, there are no reports of CSF rhinorrhea diagnosed using contrast-enhanced FLAIR imaging.High-resolution CT and MR cisternography can only diagnose the morphology and localization of the lesion.Therefore, when the lesion is small, even if there is evidence of CSF pooling into the nasal cavity, the cribriform plate may simply droop toward the nasal cavity without the dural injury complications, making it difficult to distinguish between the two.In such cases, the addition of pre-and postcontrast-enhanced FLAIR imaging may illuminate local damage and contribute to the diagnosis.Here, we demonstrate the use of pre-and postcontrast FLAIR subtraction to diagnose a case of CSF rhinorrhea that was previously ambiguous through high-resolution CT and MR cisternography.This novel approach enabled appropriate treatment and a favorable clinical outcome for the patient.

Conclusion
In addition to high-resolution CT and MR cisternography, preand postcontrast FLAIR subtraction may prove useful in imaging CSF rhinorrhea.

Patient consent
Written informed consent was obtained for patient information to be published in this article.

Fig. 1 -
Fig. 1 -Reconstruction of the initial CT with a 0.5-mm slice.The coronal image (A) and sagittal image (B) of the CT reveal a protrusion of soft density from the right ethmoid cribriform plate toward the nasal cavity (arrow).

Fig. 2 -Fig. 3 -Fig. 4 -
Fig. 2 -MR cisternography demonstrates pooling of CSF in the right nasal cavity due to the defect in the right cribriform plate of the ethmoid bone.However, the findings are minor (A, B, arrow).